Frequency Division Duplex (FDD) communication standards, such as Wideband Code Division Multiple Access (WCDMA), provide that a transmitter operates in one frequency band and the receiver operates in another. The spectral offset between the Tx and Rx frequencies (known as a duplex frequency) for FDD systems is the same for any frequency in a band, though different bands will have different duplex frequencies. Spurious frequency components from the transmit (Tx) signal can overwhelm the receive (Rx) signal, particularly in highly sensitive receivers. Some systems, such as low-band Enhanced Data rates for GSM (Global System for Mobile Communications) Evolution (EDGE) systems, isolate the receiver while a transmitter is emitting. While such isolation protects the receiver from the transmitter in the same mobile device, the noise generated during transmission of one mobile device can impact the receiver of other mobile devices.
FIG. 1 illustrates frequency spectra of an example system, such as one of those described above. The span and location of receive band 120 remains constant for all Tx frequencies 110a-110n, representatively referred to herein as Tx frequency or frequencies 110, which can change upon command in certain devices. The respective offsets 115a-115n from the Tx carrier 110 to the Rx band 120, representatively referred to herein as offset(s) 115, are thus variable and, as such, present challenges in reducing noise in receive band 120 for all Tx frequencies 110. Reducing the noise by noise shaping, for example, such as by sigma-delta modulation, would require a very high sampling rate to span a noise-shaped frequency region, i.e., 20 MHz offset 115n to 70 MHz offset 115a, sufficient to cover the full range of Tx frequencies 110. Such high sampling rate places an often unacceptable burden on system resources.